The overall goal of this project is to understand the antibody response by studying B lymphocyte activation by helper T cells in vitro. The current goal is the identification and characterization of contact-dependent helper signals delivered by the T cell to the B cell. The hypothesis, based on the results of the current grant period, is that contact- dependent help is not a direct consequence of T cell recognition of antigen on the B cell surface, but instead depends on the expression of new surface proteins on the helper T cell, including the newly discovered CD40 ligand (CD40L), a transient activation antigen whose expression is largely restricted to acutely activated CD4+ T cells, that binds the B cell differentiation antigen, CD40, and activates B cells. Like lymphokines, these new membrane proteins are tightly regulated by T cell activation. By engaging receptors on B cells, they provide the limiting helper signals that, together with the appropriate lymphokines, enable B cell proliferation and differentiation to antibody secretion. The specific aims are: 1) to determine to what extent CD40L accounts for the delivery of early and late components of contact help to the B cell; 2) to identify other molecular interaction that contribute to or modulate the effects of CD40L in contact help; and 3) to understand the effects of contact help on the B cell, and how contact help differs in its effects from signaling through the B cell antigen, receptor, membrane immunoglobulin. Soluble CD40 and monoclonal anti-CD40L will be used to measure CD40L expression and block CD40L activity. CD40L-transfected, antigen-specific hybridomas will be used to measure CD40L effects in the context of other resident and induced T cell surface molecules. CD40L-deficient T cells will be isolated and studied. information will be obtained about the effects of contact help on early activation genes expression, transcription factors, proliferation and differentiation to immunoglobulin secretion, transcriptional regulation of immunoglobulin class switching, and expression of B cell surface markers.